Incandescent mantles became a commercially practicable product following the introduction by Carl Auer Von Welsbach in 1893 of a mantle having a composition of about 99% thorium oxide and about 1% cerium oxide. This composition was determined by experimentation covering a wide range of metal oxides including rare earth elements.
Thorium is a naturally occurring radioactive metal. Its decay products include alpha and beta radiation, radium isotopes, and thoron gas, which is an isotope of radon gas. Thorium is listed by US government agencies as carcinogenic, and its processing is kept under strict governmental licensing and control.
Thorium mantles are very fragile and the shocks induced by normal usage cause rapid disintegration requiring frequent replacement. Each time a mantle is changed and the old mantle discarded, about 0.3 grams of thorium oxide, which is soft and powdery, is released into the environment in an uncontrolled manner, exposing the user and others to a potential health hazard.
Early attempts to provide alternatives to the thorium/cerium composition were focussed on a desire to circumvent the Welsbach patents rather than to eliminate thorium oxide, but once these patents expired the thorium/cerium composition was universally adopted and, except for minor proprietary variations by various manufacturers, this composition has remained virtually unchanged, since no effective alternative was considered possible or even necessary. Now, however, a need exists to reduce, or preferably eliminate, the unnecessary release of radioactive material into the global environment.
Further, a more robust incandescent mantle than the conventional thorium/cerium mantle heretofore provided would enhance the reliability and reduce the maintenance expenses of incandescent lanterns and gaslights, and permit the development of new forms of such devices, not possible with the fragile thorium mantle.
Although it has been recognized that zirconium oxide would be a stronger and non-radioactive alternative to thorium oxide as a material for incandescent mantles, a method for the practical production of mantles based on zirconium oxide has been wanting.
Encyclopedia Britanica 1 describes in some detail the early history of the incandescent mantle. On page 656 it is noted that zirconia, and yttria, were possible alternatives to thorium oxide, but they were considered unsuitable because of their fragility. Zirconia was also rejected on the basis of shrinkage and slow volatilization.
The earliest mantles were of upright form, in which the mantle was supported over a non-luminous flame by means of a wire frame. Although obsolete, this type of mantle is still manufactured for decorative purposes.
The problem of fragility was addressed in Voelker U.S. Pat. No. 546,792 which proposed an incandescent mantle fashioned from filaments of porcelanous material impregnated with solutions of rare earth salts, or with rare earth oxides incorporated in the porcelain base material. There is no record in the literature to indicate that such incandescent mantles were ever a commercial success.
A more recent attempt to address the fragility of thorium oxide mantles is disclosed in Reid et al U.S. Pat. No. 3,738,793, which has a layer of thorium and cerium oxides deposited on the outer surface of the porous ceramic element of a gas burner operating on the surface combustion principle.
The described manufacturing process is complex, it requires the deposition of several layers of metal oxides (column 2 line 29 through column 3 line 49) onto a special substrate followed by a heat treatment process, all conducted under carefully controlled conditions. There is no indication that incandescent lighting devices according to U.S. Pat. No. 3,738,793 have ever been produced on a commercial basis.
Gas Appliance and Space Conditioning Newsletter, March 1987, reviews the status of incandescent mantles and provides a reference that mantle compositions other than thorium/cerium were inferior to it in light output. Also reviewed is the device disclosed in the above mentioned U.S. Pat. No. 3,738,793 which, also, attempts to coat thorium/cerium oxide mixtures onto substrates of silicon carbide or zirconia, which are described as being the most promising materials because of excellent high temperature strength and thermal shock resistance, but these attempts are recorded as having poor success and that a sintering process instead of coating was to be tried. There is no indication in this document that the substrate materials were themselves to be used as the incandescent light emitting body.
Nernst U.S. Pat. No. 685,730, describes electric lamp glowers composed of 85% zirconia and 15% of yttria or other rare earth oxides. Nernst U.S. Pat. No. 685,732 describes electric lamp glowers composed of 80% zirconia, 10% yttria, and 10% erbia. Nernst U.S. Pat. No. 685,733 describes electric lamp glowers composed of zirconia combined with rare earth oxides derived from yttrium containing minerals in their natural-state proportions. However, to date, neither Nernst nor anyone else has described the use of zirconia, yttria and erbia in a gas mantle.
In each of the above Nernst patents, the materials were to be mixed with binding agents, compressed into bars or tubes, and fired in a furnace to produce ceramic elements termed glowers for use in Nernst lamps (see Encyclopedia Britannica, p. 669). The resulting glowers typically have thicknesses of about 1-2 mm, much greater than those of the fibers employed in gas mantles. These lamps made use of the property possessed by certain ceramic materials to be good electrical insulators when cold but good conductors when hot. After pre-heating to initiate conduction, the glower element was raised to, and maintained at, the required temperature by passage of an electric current thus rendering the element an incandescent light emitter.
Intended as an intermediate light source between the powerful carbon arc light and the weak Edison carbon filament lamp, the Nernst light was rendered obsolete by the introduction of the tungsten filament lamp, although Nernst glowers are still produced in small quantities as spectroscopic light sources. While the light emitting property of the Nernst glower does not depend on the passage of electric current, and in theory any suitable heat source can be employed to cause the materials of the glower to incandesce, use in conjunction with a gas flame is not known.
"Making Ceramics Tougher" (July 27, 1987) MACHINE DESIGN, pp. 84-89, discusses a new variety of ceramic materials based on metal oxides such as alumina, zirconia, and magnesia. These are termed transformation-toughened, or stabilised, ceramics in which a base oxide is alloyed or doped with a small percentage of other metal oxides which strengthen and stabilize the structure by reducing the metal oxide grain size, so increasing resistance to fracture.
One variety of transformation toughened ceramic is yttria-stabilized tetragonal zirconia polycrystal (Y-ZTP), which is zirconium oxide with the addition of a small percentage of yttrium oxide (8-10% for example). This ceramic has desirable strength properties, which may be improved further by the addition of a second stabilizing rare earth oxide to eliminate certain temperature degradation phenomena. It will be noted that the composition of the Y-ZTP materials bears a close resemblance to the Nernst glower materials described above, but also, that these materials have not been suggested for use in gas mantles. Another form of toughened ceramic utilizes aluminum oxide toughened with a small percentage of zirconium oxide and/or chromium oxide.
Ceramic fibers are of special value as heat insulating and refractory materials, but their preparation has been difficult due to the brittleness and high melting point of the appropriate materials. Hamling U.S. Pat. No. 3,385,915 discloses a process by which a host structure composed of a cellulosic material, such as a woven fabric, is used as a precursor to absorb dissolved metallic compounds after prior dilation of the woven fibers with plain water. The dissolved compounds enter into spaces within the microscopic crystallite structure of the dilated cellulosic fibers. A subsequent heat treatment, in an oxygen-controlled atmosphere within a special furnace, pyrolyses the organic fiber and leaves an amorphous refractory metal oxide structure in the shape of the host cellulosic material. The product ceramic fibers have tensile strength and flexibility. However, the heat treatment requires close control over temperature and the oxygen content of the atmosphere surrounding the fibers. It is emphasized in the disclosure, Column 7 line 23 through Column 8 line 5 that direct ignition of the impregnated material must be avoided because the product then becomes weak and brittle. The process of this disclosure is usually referred to as the precursor process for producing ceramic fibers.
Example 7 of the above disclosure, Column 14, line 67 through Column 15, line 19, describes application of the subject process to a thorium/cerium incandescent mantle commercially manufactured for use in gasoline lanterns. The resulting product was a thoria fiber structure having the desirable properties of strength and flexibility, which should render it suitable for incandescent mantle applications. However, discussion of this example with patentee Hamling indicated that when tested as an incandescent mantle, the light output was less than that obtained from a regular mantle, and after a period of use the amorphous structure of the thoria fibers reverted to the same crystallized form produced in a regular mantle when it is installed on a lantern burner and ignited prior to admission of fuel to the burner, as described on the mantle package.
The Zircar Product Brochure describes an end product of the precursor process which is commercially manufactured under the trade name of Zircar. One version of this utilizes zirconium and yttrium compounds, with hafnium as a second stabilizing agent, to reduce ceramic fibers. Incandescent mantles are not among the products offered by the manufacturers of Zircar.
Levy, S. I., Pitman's Common Commodities and Industries; "Incandescent Lighting", Sir Isaac Pitman and Sons Ltd.: London (1922), pages 75 through 91, describes in some detail the actual processes of incandescent mantle manufacture, which is itself a form of precursor process. A cellulosic yarn, usually rayon, is knitted into the form of a tubular webbing which is cut in sections to form precursor mantles. These sections are closed by stitching at one end, leaving the other end open. They are then impregnated by immersion in a solution of salts of thorium and cerium, usually the nitrates. Excess solution is removed and the sections are dried.
At this stage the mantle manufacture may be completed in several different ways. For use as a soft or tie-on inverted mantle for portable gasoline and similar lanterns, the open end is threaded with a length of heat resisting yarn by which means the mantle may be tied to the burner nozzle of the lantern. For use, the mantle is ignited without the fuel gas flowing. This pyrolysis burns away the rayon base yarn and converts the thorium and cerium nitrates into oxides, so forming an oxide skeleton of the original knitted structure. This oxide skeleton is very delicate and easily damaged. When the pyrolysis is complete, the fuel gas is admitted to the burner and ignited, so rendering the mantle incandescent, this also consolidates the oxide skeleton and shapes it to fit the gas flame.
For use on the burners of other types of lights, the soft mantle described above is attached to a ceramic ring provided with internal lugs which engage with projections on the nozzle of the gaslight burner. The initial pyrolysis procedure is Performed on the gaslight exactly as described above for a gasoline lantern.
Another variation of inverted mantle manufacture is based on the soft mantle and ceramic ring assembly described above, but the pyrolysis is performed during manufacture. The resulting mantle, termed a hard or pre-formed mantle, is intended for use on fixed gaslights, but since the oxide skeleton is very fragile it is dipped in a collodion (nitro cellulose) solution and dried. The nitro-cellulose so deposited strengthens the oxide structure for transportation. The collodion deposit is burned off at the consumers lamp prior to ignition of the fuel gas. Knitted fabrics are particularly suitable for the fabrication of incandescent mantles since they can be produced directly in one-piece tubular form, and they possess elasticity in both the vertical and horizontal directions, which property aids the shrinking and forming process during pyrolysis, and which property is virtually absent from woven fabrics.
There are various forms of complex knit stitch used in mantle manufacture which are intended to enhance the strength of the oxide structure. Also, plain loop knit, also termed jersey knit, or stockinette knit, is used for some soft mantles. This form of knit uses interlinking loops, each loop is identical to every other loop both horizontally and vertically. However, the great friability of thorium oxide precludes substantial strength improvements based on structural modifications. "The Formation of Loops and Construction of Looped Fabrics" TECHNICS Volume 1, pg 499, George Newnes Ltd (London 1904) describes in some detail the structure and properties of plain knit textiles.